1. Field of the Invention
The present invention generally relates to a Combination of a Circuit Switched (CS) call and an Internet protocol Multimedia Subsystem (IMS) session (CSI) interworking. More particularly, the present invention relates to a method and apparatus for handling a call request including a request for a real-time service received from a terminal using an IMS session (IMS terminal) for a real-time voice call or video call, in a terminal capable of combined CS call and IMS session simultaneously (CSI terminal).
2. Description of the Related Art
A CS scheme, which was designed for traditional voice service and real-time service, establishes a CS-based fixed communication route between one party and the other party. An IMS scheme increases transmission efficiency and ensures security by establishing an Internet Protocol (IP) packet-based non-fixed communication route. For this reason, an IMS scheme is suitable for data service, messaging, and file transfer. An IMS scheme provides multi-user connectivity as well as a simple one-to-one call. Messages and user traffic are sent by a CS call in a CS scheme, while they are sent by an IMS session in an IMS scheme.
A combination of CS and IMS services, i.e. a CSI scheme, provides a real-time service, such as voice transmission by a CS call and an enhanced Packet-Switched (PS) service, such as messaging, file transfer, and the like, by an IMS session. A CSI-capable terminal, i.e. a CSI terminal can use an IMS session for a non-real-time PS service, simultaneously with a CS call for a real-time service.
FIG. 1 shows a CSI-supporting mobile communication network according to the prior art. A Universal Mobile Telecommunication Service (UMTS) mobile communication network based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS) is shown as supporting the CSI service.
Referring to FIG. 1, the CSI-supporting mobile communication network includes a Radio Access Network (RAN) 104 and a Core Network (CN) 106. The RAN 104 connects a User Equipment (UE) 102 to the CN 106. The terms “UE” and “terminal” are interchangeably used in the same sense. As configurations of the RAN 104 according to radio access technologies are known, its detailed description is not provided herein.
The CN 106 includes a CS domain 114, a PS domain 108, and an IMS domain 122. The CS domain 114 has entities supporting CS calls. Major ones of the CS entities are a Mobile Switching Center (MSC) 120 and Media GateWays (MGWs) 116 and 118. The MSC 120 processes incoming and outgoing calls of Node Bs within the RAN 104 and provides central control to the Node Bs, for the Node Bs' efficient operation. The MGWs 116 and 118 are gateways supporting bearer traffic and control traffic.
The PS domain 108 supports packet-based services and includes network nodes, such as a Serving GPRS Support Node (SGSN) 110 and a Gateway GPRS Support Node (GGSN) 112. The SGSN 110 manages information about the locations of UEs and performs security-related functionalities and access control. The GGSN 112 is an entity of a GPRS network, serving as a radio gateway between the SGSN 110 and an external network 130 like a Public Switched Telephone Network (PSTN), an Integrated Service Data Network (ISDN), a Packet Data Network (PDN), or the like. The UE 102 can access the external network 130 via the GGSN 112.
The IMS domain 122 includes entities supporting IMS sessions. Especially these entities include a Serving-Call Session Control Function (S-CSCF) 124, an Application Server (AS) 126, and a Media Gateway Control Function/Media Gateway (MGCF/MGW) 128. The S-CSCF 124 establishes an IMS session and processes a Session Initiation Protocol (SIP) message received from the UE 102. To provide a service that the UE 102 requests, the S-CSCF 124 sends the SIP message to other entities in the IMS domain 122 so the IMS session is completed. The AS 126 is an entity for providing a variety of applications to users, including e-mail or Push to talk over Cellular (PoC). The MGCF/MGW 128 is a network node for enabling communications between an IMS user and a CS user, and includes an MGCF and an MGW. For interworking between an IMS network and a legacy network, for example, between the PSTN and the CS domain 114, the MGCF is responsible for signal conversion and the MGW is responsible for media conversion.
Besides the above-described entities, other entities may exist in each domain and entities in the domains may be involved in the CSI service.
FIG. 2 shows a flow of control signals between CSI UEs for a typical CSI service. A first CSI UE 204 (CSI UE 1) and a second CSI UE 214 (CSI UE 2) are connected to a first network 202 (network 1) and a second network 212 (network 2), respectively. Network 1 includes a first PS domain 206 (PS domain 1), a first CS domain 208 (CS domain 1), and a first IMS domain 210 (IMS domain 1). Network 2 includes a second PS domain 216 (PS domain 2), a second CS domain 218 (CS domain 2), and a second IMS domain 220 (IMS domain 2).
Control signals 224 associated with a CS call for speech between CSI UE 1 and CSI UE 2 are sent over CS domain 1 and CS domain 2. Control signals 222 associated with an IMS session for a multimedia service are sent over PS domain 1, IMS domain 1, IMS domain 2, and PS domain 2. The IMS session can be controlled by the SIP. The SIP is a protocol developed by the Internet Engineering Task Force (IETF) Multiparty Multimedia Session Control (MMUSIC) Working Group as an alternative to H.323. The SIP equips platforms to signal the setup of voice and multimedia calls over IP networks.
If both UEs participating in communications are CSI-capable, control flows are executed separately over their domains. However, if both of the UEs are not CSI-capable, for example, either of the UEs is IMS-capable only, the above control flows are not viable. If a CSI UE moves to an area where no CS calls are available, for example, only PS services are available, the CSI-UE should implement a real-time service in an IMS session despite its CSI capability. Accordingly, there is a need for a technique for enabling communications between a CSI UE and an IMS UE, taking into account a network where the CSI UE is located.